Filter holders with a glass panel are known. In this prior art, a glass panel ground to optical quality and usually coated with an antireflection layer presses the filter made of paper or plastic fabric level or flat onto the filter holder. This is caused either by the weight of the glass panel and the frame, by screw connection, or spring force. This solution is disadvantageous because over time, particularly during the cleaning process, the particles scratch the glass panel which then has to be replaced at great cost. It is further disadvantageous because particles from the filter remain on the glass panel when said panel is removed and a reexamination of the filters yields different results than the results obtained from the first observation. Furthermore, insufficient cleaning poses the risk of adhering particles contaminating the filter to be examined next and thus falsifying the measurement result. However, it is most disadvantageous that filters covered with a glass panel cannot be imaged with an electron microscope and also do not allow for an elemental analysis with X-ray spectroscopy.
Furthermore, filter holders with a clamping ring are known. In this prior art, the filter is retained at the periphery with a tension ring. This is effected either by the weight of the ring, by screw connection, or spring force. This is disadvantageous because the filter material becomes uneven due to the drying process after filtering. The standard microscopic examination is commonly carried out with objectives 5× and 10×. Their low depth of field frequently requires refocusing, particularly in automated analysis systems with a motorized scanning table and a motorized autofocus. In unfavorable cases, the time for the analysis is at least doubled. One advantage of this arrangement over the filter holder with a glass panel is that the filters can be imaged with an electron microscope and an elemental analysis using X-ray spectroscopy is also possible.